docs: def check(self, url: str) -> Optional[dict]:
          """Check if data for a url has expired.
  
          Data is not fetched again if it has expired.
  
          Args:
              url: url to check expiration on
  
          Returns:
              value of the data, possibly None
          """
          data = self.data.get(url)
          if data:
              data = self._check_expiration(url, data)
          return data.data if data else None